Add C++ support

[i-ky]

I've tried to enter the following code:

class Foo {};

int main()
{
  return 0;
}

... and I'm getting:

Error running /usr/bin/clang-6.0 -I /home/klee/klee_src/include -emit-llvm -c -g /tmp/code/code.c -o /tmp/code/code.o:
/tmp/code/code.c:1:1: error: unknown type name 'class'
class Foo {};
^
/tmp/code/code.c:1:10: error: expected ';' after top level declarator
class Foo {};
         ^
         ;
2 errors generated.

Please add an option to select C++ compiler.

[ccadar]

Thank you for reporting this. We are not actively extending KLEE-Web at the moment, but we would welcome such a PR.